I. Field of the Invention
The present invention relates generally to the field of spinal fixation surgery, and more specifically to posterior cervical fixation assemblies and techniques for securing an orthopedic rod to a spine.
II. Background
The spinal column is a highly complex system of bones and connective tissues that provides support for the body and protects the delicate spinal cord and nerves. The spinal column includes a series of vertebral bodies stacked one atop the other, each vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces exerted upon the spinal column. A vertebral canal containing the spinal cord is located behind the vertebral bodies.
There are many types of spinal column disorders including scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine), excess lordosis (abnormal backward curvature of the spine), spondylothesis (forward displacement of one vertebra over another), and other disorders caused by abnormalities, disease or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain, as well as diminished nerve function.
Surgical techniques commonly referred to as spinal fixation use surgical implants for fusing together and/or mechanically immobilizing two or more vertebral bodies of the spinal column. Spinal fixation may also be used to alter the alignment of adjacent vertebral bodies relative to one another so as to change the overall alignment of the spinal column. Such techniques have been used effectively to treat the above-described conditions and, in most cases, to relieve pain.
One spinal fixation technique involves immobilizing the spine using orthopedic stabilizing rods, commonly referred to as spine rods, which run generally parallel to the spine. This may be accomplished by exposing the spine posteriorly, and fastening bone screws and/or hooks to the vertebral bodies. The screws and/or hooks are generally placed two per vertebra and serve as anchor points for the spine rods. Clamping or coupling elements adapted for receiving a spine rod therethrough are then used to join the spine rods to the screws or hooks. The aligning influence of the spine rods forces the spinal column to conform to a more desirable shape. In certain instances, the spine rods may be bent to achieve the desired curvature of the spinal column.
There are many disadvantages associated with current spinal fixation devices. For example, many prior art bone fixation devices are less than optimal for capturing spine rods when the coupling elements must be rotated to extreme angles. With such devices, pivotal movement of the anchor portion is limited to an angle of generally no more than 40° (measured from vertical) in any direction. Surgeons have encountered considerable difficulty attempting to insert spinal fixation devices when the coupling elements are out of alignment with one another due to curvature of the spinal column and the different orientation of adjacent pedicles receiving screws. As a result, spine rods must often be bent in multiple planes in order to pass the rods through adjacent coupling elements. This may potentially weaken the overall assembly and results in longer operations and a greater likelihood of complications. Further problems may arise when applying an occipital plate due to the natural curvature of a patient's spine, the difference in relative position of the occiput compared with the vertebral bodies, and the irregular shape and thickness of the occiput.
The occipital-cervical (OC) junction is the most cephalad portion of the spinal column, spanning from the occiput to the C2 vertebra. This bony junction allows for significant mobility while maintaining biomechanical stability. OC instability is a rare disorder with potentially life-threatening consequences. Such instability may cause disabling pain, cranial nerve dysfunction, paralysis, or even sudden death. One common effect of OC instability includes dislocation of the atlantooccipital (i.e. C1-occiput) joint and complex fractures of the C1 and C2 vertebrae. OC fusion is often warranted when the OC junction is rendered unstable. OC fixation procedures to stabilize the OC junction are a challenge to spine surgeons. Due mostly to the neighboring anatomy and relatively difficult occipital bone purchase, multiple attachment points to the occipital bone are generally required to increase construct rigidity. Moreover, the instrumentation utilized must accommodate the anatomic structures and satisfy the biomechanical needs and the kinematics of the area. Current systems are generally rigid posterior fixation systems using rods/screws or plates, providing biomechanical stability and generally high rates of fusion. However, current OC fixation systems have limitations in the cephalad part of the construct. Part of the problem is that the occiput does not easily accommodate instrumentation, and the area available for the fixation of implants is limited. Occipital screws required in current techniques may also be associated with the potential for intracranial injuries.
The occipital condyles are the only osseous structures that support the head, with a length generally ranging from 16.7 mm to 30.6 mm, a width generally ranging from 6.5 mm to 15.8 mm, and a height generally ranging from 5.8 mm to 18.2 mm. The condyles are generally oval shaped and converge ventrally with a sagittal angle ranging from 10 to 54 degrees. The variability in the anatomic parameters requires a careful radiologic analysis of the condyles before screw placement. The hypoglossal canal, which passes above the occipital condyle, is surrounded by the jugular tubercle superiorly, the jugular foramen superolaterally, the sigmoid sinus laterally, and the occipital condyle inferiorly, with the axis 45 degrees in the axial plane and directed slightly superiorly. The mean distance from hypoglossal canal to the inferior border of the condyle is 11.5 mm which allows enough room to safely place a condylar screw. Moreover, the canal offers additional protection to the hypoglossal nerve as it is surrounded by cortical bone. The proximity of neurovascular structures during the surgical exposure is another anatomic consideration that must be taken into account prior to performing any occipital procedure.
The present invention is directed at overcoming, or at least improving upon, the disadvantages of the prior art.